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STUT AND NUTS
Unified Thread StandardFrom Wikipedia, the free encyclopedia
The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances,
tolerances, and designations—for screw threads commonly used in the United States and Canada. It has the
same 60° profile as the ISO metric screw thread used in the rest of the world, but the characteristic dimensions
of each UTS thread (outer diameter and pitch) were chosen as an inch fraction rather than a millimeter value.
The UTS is currently controlled by ASME/ ANSI in the United States.
Contents
[hide]
1 Origins
2 Basic profile
3 Designation
4 Gaging
5 Tolerance classes
6 Related standards
7 See also
8 Notes
o 8.1 Bibliography
9 External links
[edit]Origins
See the "History of standardization" section of the screw thread article .
[edit]Basic profile
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The basic profile of all UTS threads is the same as that of allISO metric screw threads. Only the commonly used
values for D maj and P differ between the two standards.
Each thread in the series is characterized by its major diameter D maj and its pitch, P . UTS threads consist
of a symmetric V-shaped thread. In the plane of the thread axis, the flanks of the V have an angle of 60° to
each other. The outermost 0.125 and the innermost 0.25 of the height H of the V-shape are cut off from
the profile.
The pitch P is the distance between thread peaks. For UTS threads, which are single-start threads, it is
equal to the lead, the axial distance that the screw advances during a 360° rotation. UTS threads do not
usually use the pitch parameter; instead a parameter known as threads per inch(TPI) is used, which is
the reciprocal of the pitch.
The relationship between the height H and the pitch P is found using the following equation:[1]
In an external (male) thread (e.g., on a bolt), the major diameter D maj and the minor
diameter D min define maximum dimensions of the thread. This means that the external thread must
end flat at D maj, but can be rounded out below the minor diameter D min. Conversely, in an internal
(female) thread (e.g., in a nut), the major and minor diameters are minimum dimensions, therefore the
thread profile must end flat at D min but may be rounded out beyond D maj.
The minor diameter D min and effective pitch diameter D p are derived from the major diameter and pitch
as
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[edit]Designation
The standard designation for a UTS thread is a number indicating the nominal (major)
diameter of the thread, followed by the pitch measured in threads per inch. For diameters
smaller than ¼ inch, the diameter is indicated by an integer number defined in the standard,for all other diameters, the inch figure is given.
This number pair is optionally followed by the letters UNC, UNF or UNEF if the diameter-
pitch combination is from the ―coarse‖, ―fine‖ or ―extra fine‖ series, and may also be followed
by a tolerance class.
Example: #6-32 UNC 2B (major diameter: 0.1380 inch, pitch: 32 tpi)
See also: drill and tap size chart
Unified Screw Threads - UNC, UNF and UNEF [2]
Major diameter
[in (mm)]
Threads per inch (Pitch)Tap drill size - Preferred
sizes[3]
Coarse
(UNC)
Fine
(UNF)
Extra fine
(UNEF)Coarse Fine Extra fine
#0 = 0.0600
(1.5240) –
80 3/64 in
#1 = 0.0730
(1.8542)64 72 #53 #53
#2 = 0.0860
(2.1844)56 64 #50 #50
#3 = 0.0990(2.5146)
48 56 #47 #45
#4 = 0.1120
(2.8448)40 48 #43 #42
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#5 = 0.1250
(3.1750)40 44 #38 #37
#6 = 0.1380
(3.5052) 32 40 #36 #33
#8 = 0.1640
(4.1656)32 36 #29 #29
#10 = 0.1900
(4.8260)24 32 #25 #21
#12 = 0.2160(5.4864)
24 28 32 #16 #14
1/4 = 0.2500
(6.3500)20 28 32 #7 #3
5/16 = 0.3125
(7.9375)18 24 32 F I
3/8 = 0.3750(9.5250)
16 24 32 5/16 in Q
7/16 = 0.4375
(11.1125)14 20 28 U 25/64 in
1/2 = 0.5000
(12.7000)13 20 28 27/64 in 29/64 in
9/16 = 0.5625
(14.2875)12 18 24 31/64 in 33/64 in
5/8 = 0.6250
(15.8750)11 18 24 17/32 in 37/64 in
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3/4 = 0.7500
(19.0500)10 16 20 21/32 in 11/16 in
7/8 = 0.8750
(22.2250) 9 14 20 49/64 in 13/16 in
1 = 1.0000
(25.4000)8 12 20 7/8 in 59/64 in
The following formula is used to calculate the major diameter of a numbered screw greater
than or equal to 0: Major diameter = Screw # × 0.013" + 0.060". For example, a number 10
calculates as: #10 × 0.013" + 0.060" = 0.190" major diameter.
A Unified Miniature screw thread series is defined in ANSI standard B1.10, for fasteners of
0.3 to 1.4 millimetres (0.0118 to 0.0551 inch) diameter. These sizes are intended
for watches, instruments, and miniature mechanisms and are interchangeable with threads
made to ISO Standard 68.[4]
These screw sizes are denoted by multiple zeroes, i.e., #000.
The formula for number sizes smaller than size #0 is given by Major diameter = 0.060" - Zero
size × 0.013", with the zero size being the number of zeroes after the first. So a #00 screw is
.047" dia, #000 is .034" dia, etc.
The number series of machine screws once included more odd numbers and went up to #16
or more. Standardization efforts in the late 19th and the early part of the 20th century
reduced the range of sizes considerably. Now, it is less common to see machine screws
larger than #14, or odd number sizes other than #1, #3 and #5. Even though #14 and #16
screws are still available, they are not as common as sizes #0 through #12.[citation needed ]
Sometimes "special" diameter and pitch combinations (UNS) are used, for example a
0.619 in (15.7 mm) major diameter with 20 threads per inch. UNS threads are rarely used for
bolts, but rather on nuts, tapped holes, and threaded OD's. Because of this UNS taps are
readily available.[5][6]
Most UNS threads have more threads per inch than the correlating UNF
or UNEF standard, therefore they are often the strongest thread available.
[7]
Because of thisthey are often used in applications where high stresses are encountered, such as machine
tool spindles[8] or automotive spindles.[9]
[edit]Gaging
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A screw thread gaging system comprises a list of screw thread characteristics that must be
inspected to establish the dimensional acceptability of the screw threads on a threaded
product and the gage(s) which shall be used when inspecting those characteristics.
Currently this gaging UTS is controlled by:
ASME/ANSI B1.2-1983 Gages And Gaging For Unified Inch Screw Threads
ASME/ANSI B1.3-2007 Screw Thread Gaging Systems for Acceptability: Inch and
Metric Screw Threads (UN, UNR, UNJ, M, and MJ)
These standards provide essential specifications and dimensions for the gages used on
Unified inch screw threads (UN, UNR, UNJ thread form) on externally and internally threaded
products. It also covers the specifications and dimensions for the thread gages and
measuring equipment. The basic purpose and use of each gage are also described. It also
establishes the criteria for screw thread acceptance when a gaging system is used.
[edit]Tolerance classes
A classification system exists for ease of manufacture and interchangeability of fabricated
threaded items. Most (but certainly not all) threaded items are made to a classification
standard called the Unified Screw Thread Standard Series. This system is analogous to
the fits used with assembled parts.
Classes 1A, 2A, 3A apply to external threads; Classes 1B, 2B, 3B apply to internal threads.
Class 1 threads are loosely fitting threads intended for ease of assembly or use in a dirty
environment.
Class 2 threads are the most common. They are designed to maximize strength considering
typical machine shop capability and machine practice.
Class 3 threads are used for closer tolerances.
Thread class refers to the acceptable range of pitch diameter for any given thread. The pitch
diameter is indicated as Dp in the figure shown above. There are several methods that are
used to measure the pitch diameter. The most common method used in production is by wayof a Go/no go gauge.
[edit]Related standards
ASME/ANSI B1.1 – 2003 Unified Inch Screw Threads, UN & UNR Thread Form
ASME/ANSI B1.10M – 2004 Unified Miniature Screw Threads
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ASME/ANSI B1.15 – 1995 Unified Inch Screw Threads, UNJ Thread Form